1. Field of the Invention
The present invention relates to an aligner, commonly known as a "stepper", which performs stepping and repeating in sequence to align and project an electronic circuit pattern on the surface of a reticle onto the surface of a wafer via a projection optical system, and an aligner, commonly known as a "scanner", which performs stepping and scanning, in sequence, to execute the alignment and the projection onto the surface of a wafer in the same manner in the manufacturing process of semiconductor devices such as ICs and LSIs. More particularly, the present invention relates to a projection aligner and a projection aligning method for manufacturing semiconductors, the projection aligner and the projection aligning method being provided with a function and a step, respectively, for achieving integral control of various influences caused by the heat of exposure light generated during a projecting and aligning process, and for making necessary corrections, such influences including the thermal deformation of a reticle, and the thermal deformation and/or a change in the refractive index of each lens element.
2. Description of The Related Art
With the recent increasing trend toward microscale patterns of ICs, LSIs, and other semiconductor integrated circuits, there has been a strong demand for further improvement in the resolution, overlapping accuracy, and throughput of projection aligners. Today, more LSI manufacturers are employing high-resolution steppers for critical layers while employing steppers that have a lower resolution, but have a wider field angle and higher throughput for noncritical layers, so as to improve throughput in their mass-production lines. In order to successfully cope with the mix and match processes required to handle such different types of steppers, it is especially important to improve the overlapping accuracy. Improving the overlapping accuracy requires that the changes in the magnification, distortion, etc., in a shot be minimized, in addition to the need for restraining the shift of a shot array in a wafer, magnification, and rotational components in a wafer. One of the factors responsible for the foregoing changes in the shot has conventionally been considered to be due to a phenomenon wherein a projection lens assembly absorbs exposure light and the respective lens elements thereof incur thermal deformation, causing the refractive index therein to be changed.
Recently, however, it has been revealed that the changes in the magnification, distortion, etc., in a shot cannot be explained simply by the absorption of exposure light by the projection lens assembly.
The problems caused by the heat from exposure light are roughly divided into the following three types:
First, as previously mentioned, when the projection lens assembly absorbs exposure light, it incurs thermal deformation and a change in the refractive index thereof, resulting in a change in the image forming performance thereof. Conventional corrective measures for coping with this problem include one wherein a sealed space is provided between particular lens elements to control and adjust the pressure in the space, or a particular lens element is moved along an optical axis to make corrections.
Second, a reticle absorbs exposure light and incurs thermal deformation, generating a magnification component. This problem has become noticeable as the technology in the industry has advanced, which is represented by exposure lamps having higher luminance, or three-layer Cr surfaces for preventing the flare of an optical system, in order to respond to the recent demand for higher throughput. To solve this problem, there has been a method in which the rise in temperature is prevented by subjecting a reticle to forced air cooling, or a method disclosed in Japanese Unexamined Patent Publication No. 4-192317 in which the thermal deformation of the reticle is determined by calculation or actual measurement and the amount of correction to be made by a lens magnification correcting means is computed according to the obtained thermal deformation, so as to make a correction.
The third problem is that a wafer absorbs exposure light applied thereto and thermally expands. This problem is conspicuous especially with a high-throughput type stepper supplying a high level of energy to the wafer per unit time. It is not easy to devise an active corrective method to solve this problem because the magnification actually generated on the wafer is strongly influenced by an adjacent shot and the generated magnification component varies depending on the position of the shot. For this reason, a method has been used whereby the heat conduction between a wafer and a chuck is improved so that the heat absorbed at a shot position on the wafer is quickly diffused to the wafer chuck, or a static measure has been proposed wherein the chuck is provided with a water-cooling device.
The foregoing conventional techniques, however, have the following shortcomings:
The errors including the magnification and the distortion component in a shot attributable to the heat of exposure light are considered to be caused by the linear sum of the aforesaid factors, namely, the reticle factor, the projection lens factor, and the wafer factor. The corrective measures, however, have been made independently for each of the three factors. In other words, no integrated corrective function has been devised for an entire stepper system. More specifically, whereas the magnification and distortion that take place on an exposure image surface involve all three of the foregoing factors, inseparably, the magnification that has occurred is regarded as being wholly due to the absorption of exposure light by a projection lens and a correction is made accordingly. Hence, a corrective operation leads to an over-correction or an under-correction depending on the individual actual processing conditions, thus preventing an effective overall corrective operation from being accomplished.